Radioreceiver



ec. 10, 1935. H. A. WHEELER Er AL 2,024,017

RADIORECEIVER Filed Nov. '7, 1933 2 Sheets-Sheet l INVENTORS HAROLD A. VWHEELER ELSON P, 6/ 65 BY m fim, V ATTORNEYS ?atented Dec. 10, 1935 PATENT OFFICE RADIORECEIVER Harold A. Wheeler,

Great Neck, and Nelson 1.

Case, Bayside, N. Y2, assignors to Hazeltine Corporation Application November 7, 1933, Serial No. 696,990 22 Claims. (Cl. 250-20) This invention relates to the reception and selection of modulated carrier signals, particularly carrier frequency waves modulated by the voice, music, and the like, and it has for its principal objects to facilitate the selection of such signals and to improve the fidelity of reception thereof.

A radio broadcast signal is ordinarily transmitted on a carrier wave having two sidebands of modulation, which are about 6 kilocycles in width on either side of the carrier. Under present radio broadcast operating conditions, the different carrier frequencies are allocated at various positions throughout the broadcast frequency range, usually 10 kilocycles apart, and in many instances the sideband frequencies of one signal channel either overlap those of an adjacent signal channel or else closely encroach upon them. In either case it is difficult, when tuning a radio broadcast receiver to a desired signal in one such channel, substantially to eliminate interference due to signals in adjacent channels, particularly when such interfering signals are intercepted at the antenna or other collector with a strength comparable to, or exceeding, that of the desired signal. In addition to interfering signals, static and other socalled background noise may interfere with quiet operation.

Quiet operation during reception under such circumstances, requires that the selecting system shall select a sufiiciently narrow band of modulation frequencies to prevent appreciable passage of interfering signals and noise. Narrowing the selected band in this manner, however, tends to impair the fidelity of reception of the signals (voice, music and the like), since the outer sidebands corresponding to the higher audio frequencies of modulation are suppressed. Accordingly, it is desirable that the selected band width be allowed to remain narrow onlywhen excessive interfering signals or noise are present, but in their absence, the selecting system. should be adjusted to freely admit and pass all of the received sidebands of the desired signal.

This latter result is attained in accordance with the present invention by the provision of an' adjustable band-pass selector, wherebythe selected band width may be expanded or contracted at will, while maintaining exact tuning of the selector to the desired carrier frequency. A feature of the invention resides in the provision of a single knob or other single control means capable of undergoing a plurality of control motions, i. e., having at least two degrees of freedom or two modes of motion which independently perform two functions, (1) tuning, and (2) expanding or contracting the band width of the band-pass selector independently of tuning. The first manipulation thus serves to adjust the tuning and the second the selectivity and resulting 5 fidelity. The said control means is therefore adapted to be operatively associated both with the tuning mechanism of the selector and with the band-width adjusting means of the selector.

The circuit and apparatus for adjusting the band width may be of any suitable type. In the preferred embodiment described hereinafter, the apparatus for adjusting the width of the band comprises double-tuned selecting circuits, the coupling between the two tuned portions thereof being variable by a single knob.

The single control means is preferably so' arranged that it' is adapted to operate the tuning mechanism only when the selector band width is adjusted to a minimum value. In this way the broadcast listener or operator is constrained to tune the selector only while the band width is adjusted to a minimum value, and hence, he is constrained to tune it accurately to the desired signal-carrier frequency. After the selector is exactly tuned, the single control means may be disassociated from the tuning mechanism and then utilized to expand the selector band width to secure any desired degree of fidelity of recep-. tion. Hence, the arrangement may be termed an expanding selector.

Another feature resides in'the provision of another or second control knob for efiecting manually both volume level control and tone control with respect to the signal from the loud speaker or reproducer. It is preferable that this second control knob, while adjusting the volume level, operate in the same degree of freedom which is characteristic of the selector knobwhile tuning" the selector. And, in performing its other function, the second control knob may also be utilized to expand or contract the tone or audio-frequency range by operation in the other degree of freedom which is characteristic of the selector knob 4 while expanding or contracting the band width. 5

Since the band width of the selector and the selectivity of the selector are reciprocally related, the control means for adjusting the band width of the selector is likewise a means for oppositely adjusting the selectivity. The resulting fidelity of reception is improved in proportion to the expansion of the selector band width.

In the accompanying drawings:

Fig. 1 is a schematic circuit diagram of a tunable modulated-carrier radio receiver adapted to receive modulated carrier-frequency signals, and embodying the expanding selector control arrangement and a combined volume and tone control arrangement in accordance with a preferred embodiment of the present invention;

Fig. 2 is a plan view of a mechanical arrangement adapted to be utilized in a radio broadcast receiver including the circuit of Fig. 1; 1

Fig. 3 is a sectional view in the plane 33 of Fig. 2; and

Fig. 4 is a sectional view in the plane 4+4 of Fig. 2.

Fig. 1 is a schematic circuit diagram embodying the invention as applied to a form of superheterodyne radio receiver. In such a receiver the signals are collected by an antenna-ground system H-IZ. A preselector system l3 is coupled to the antenna-ground system through an antenna coupling transformer I4 comprising pri mary coils l5 and I6 and a secondary coil I1. There is inductive coupling between coils l6 and I7; and between coils l1 and I5. There is also inherent capacitive coupling provided between coils I1 and I5. The transformer I4 is more fully described in United States Patent 1,907,916, of Harold A. Wheeler, granted May 9, 1933.

The preselector I3 is composed of two tunable circuits, the first of which comprises secondary coil I! and its associated variable tuning condenser l8, and the second of which comprises-a secondary coll I9 and its associated variable tuning condenser 20. These two tunable preselector circuits are coupled together by a coupling coll 2| which acts-as a primary coil with respect to secondary coil I9, and they are also coupled by a condenser 22 connected between ground and the junction of coils l9 and 2|. This type of double-' tuned preselector coupler in the manner just explained, is described in .United States Patent 1,927,672, of Harold A. Wheeler, granted September 19, 1933.

It is well known that when the inductive coupling between two tuned circuits, such as the two tunable circuits of the preselector, is increased, the admission band width of the coupling system is increased without shifting the center frequency of the band. Such a coupling system is a band-pass selective network, or band selector. When the inductive coupling is increased beyond a certain amount, the characteristic curve of the band-pass selector develops a double peak. The maximum value of coupling which develops only a single peak is known as the optimum value. Reducing the coupling below optimum reduces the band width of the single-peak characteristic until a limiting minimum value is reached. Increasing the coupling above optimum increases the frequency separation of the two peaks of the characteristic, and therefore the band width, but it also makes the individual peaks sharper. This type of band-pass selector is most useful when the inductive coupling is between half and double the optimum value. The selectivity of band-pass selectors is inversely proportional to the band width. This effect is utilized by making the coupling coil 2| movable axi ally with respect to its associated secondary coil |9, as indicated by the double-ended arrow drawn through coil 2|. Consequently, the selectivity, or band-width, of the preselector may be adjusted at will by moving the coupling coil 2| with re-' spect to coil 9.

The output of the preselector is'im'pressed upon an oscillator-modulator tube 24 through a tunable transformer 25. The primary coil 26 of the transformer is resonated by its associated condenser 21 to a frequency somewhat below the tuning range, and the secondary 28 is tunable by its associated variable tuning condenser 29. A fixed condenser 30 is connected in the tuned circuit to improve the alignment of this tunable circuit with those of the preselector, as described in the said Patent No. 1,927,672.

The oscillator-modulator tube 24 is of the socalled 2A7 type, similar to that described in the copending application of Harold A. Wheeler, S rial No. 654,327, filed Jan. 30, 1933; and the circuit thereof is of a type described in the copending application of Harold A. Wheeler, Serial No. 654,326, filed Jan. 30, 1933. The tube 24 comprises a cathode 3|, an inner grid 32, an inner anode 33, an outer grid 34, a double screen and an outer anode 36. The oscillator portion comprises a circuit connection between the inner anode'33 and ground, thiscircuit including a I coil 31 and a coil 38 in series with a radio-frequency bypass condenser 39. Coil 3! is shunted by an adjustable condenser 40 for aligning the oscillator at lower frequencies, this condenser re maining fixed after it is finally'adjusted, and serving to resonate the coil 31 at a frequency below the tuning range of the oscillator.

The coils 31 and 38 of this anode circuit are coupled to the inner-grid circuit, 4|4243, by an inductive coupling to coil 4|, part of which is connected between the inner grid 32 and ground. Coil 4| is shunted by an oscillator tuning condenser 42 and a fixed condenser 43 in series therewith, the latter serving to limit the oscillator frequency range. The cathode circuit is completed to ground by a resistor 44 and a bypass condenser 45. Oscillations are produced by the feedback coupling between the inner-anode circuit and inner-grid circuit. The oscillation voltage across coil 4| is held nearly uniform over the tuning range by propbrtioning the feedback couplings to coils 31 and 38, the former having more effect at lower frequencies and the latter at higher frequencies.

The radio-frequency signal applied to the outer signal grid 34 from the amplifier 23 is modulated in tube 24 by the oscillations, and the resulting intermediate-frequency product of the modulation appears in the outer-anode circuit of tube 24 The output of tube 24 is coupled by a doubletuned transformer 46 to an intermediate-frequency amplifier 41 of the pentode type. The primary coil 48 of transformer 46 is tuned by a condenser 49 to the intermediate frequency, and the secondary coil 50 is tuned to the intermediate frequency by a condenser 5|. Condensers 49 and 5| are each adjustable, but when finally adjusted remain fixed. The primary coil 48 is movable toward and away from the secondary coil 50, simultaneously with like movements of coil 2| with respect to coil l9, and coil 55 with respect to coil 56. Such movement of coil 48 with respect to coil 50 permits adjustment of the band width of the fixed-tuned intermediate-frequency selecting system, in'the manner described above in connection withthe radio-frequency preselector. Condensers 49 and 5| are permanently tuned to the intermediate frequency under the condition of minimum coupling between the coils 48 and 50, in order to avoid errors due to the double-peak characteristicproduced when the coupling exceeds optimum.

The output of amplifier tube 47 is coupled to amplifier tube 53 through a double-tuned intermediate-frequency transformer 53. The latter, which is similar in every essential respect to tuned transformer 46, includes primary and secondary coils 55 and 56, respectively, and tuning condensers i and 58 therefor, respectively.

The tube 53 is a composite tube of type 213'? comprising a pentode intermediate-frequency amplifier and a diode rectifier. The diode rectifier elements are the cathode 59 and the diode anode 60. The cathode 59, the control grid 52, the screen Si, the suppressor 62 and the anode 63 are the electrodes of the pentode amplifier. The intermediate-frequency signal is amplified in the amplifier portion of tube 53, and conducted to a double-tuned intermediate-frequency transformer t4 generally similar to transformers 56 and 5 3. Transformer M comprises primary and secondary coils 65 and 66, tuned by condensers ti and 68, respectively.

The output of transformer 66, across inductance 66, is impressed between the diode anode 60 and cathode 59 through a condenser 53f. The diode operates as a rectifier and develops across resistor 09 both direct and audio-frequency voltages, which latter are impressed on the control grid id of an audio-frequency amplifier ll through a resistor 72 and a coupling condenser 33. There is connected in a path between the control grid it and ground, condenser i i and resistance it in series. The function of this path is to attenuate to a certain extent the higher audio frequencies, thus compensating for the accentuation of the sidebands corresponding to such higher audio frequencies in the intermediate-frequency double-tuned transformers as when the resultant band selection characteristic of the latter is adjusted to develop a double peak. Resistor it and condenser ll function as a grid leak and an audio-frequency bypass condenser, respectively.

The output of tube H includes, in series, a resistor 78, an audio-frequency coupling condenser l9 and a variable attenuator 88. The input terminals of an audio-frequency amplifier system 36 are shown connected to the variable attenuator, which is the volume level control. This audio system includes any desired conventional apparatus such as a power amplifier and a loudspeaker. A variable resistor $2 is shown to represent a generalized tone control connected within the audio system in any of several well known arrangements. For example, the variable resistor 32 may be connected in series with a condenser and this series path may be shunted across the input or output terminals of a tube.

There is associated with the receiver a system of automatic volume control which functions to maintain the response of the receiver substantially constant over a wide range of received signal intensities. This automatic volume control comprises a tube 83 of type 2137, which is a combined pentode amplifier and diode rectifier like tube 53. The intermediate-frequency output voltage of the coupling transformer 56 is impressed on the amplifier control grid 86. The amplified signals appearing in the circuit of anode 85 are coupled by a double-tuned intermediate frequency transformer 86 to the diode anode 8h The coupling transformer 88 comprises primary coil 88 tuned by condenser 88, and secondary coil 90 tuned by condenser 9 i. This transformer possesses a bandpass characteristic which is much wider than that of transformers 46 and 54. A rectified uni-directional voltage responsive to the intermediatefrequency carrier is developed across resistors 92 and 93 connected in series between the cathode 94 and the secondary coil 90. 5

The uni-directional voltage across resistors 92 and 93 is impressed on the control grids oftubes 23, 2d, and ill, by means of the connections including resistors 95, 96, and 91.

For the purpose of facilitating and obtaining the proper tuning adjustment, which would otherwise be somewhat difficult owing to the typical action of the automatic volume control arrangement, a visual tuning meter ea, or other visual tuning indicator, is connected in the cathode lead of the first radio-frequency ampli- Since the anode current of the tube meter, an indication of minimum direct current is that of accurate tuning.

There is also provided a connection 99 from a point between resistors s2 and 98 to the grid circuit of audio amplifier it to control or vary the gain thereof. This improvement in automatic volume control is disclosed in the copending application of Harold A. Wheeler, Serial No. 603,500, filed April 6, 1932.

The tubes 23, 2Q, t'll, and it, have control grids of the gradual cutofi type. Such a control grid characteristic improves the automatic volume control operation.

There are provided throughout the receiver, resistors and bypass condensers, as shown, to improve the general operation. Although batteries are used to designate direct voltage sources 5 throughout the receiver, it should be understood that these are only symbols, and may represent,

for example, a single source of rectified alternating-current source with the usual ripple filter, bypass condensers and voltage-dividing resistors. 4

It is noted that the double-ended arrows denote movable or adjustably coupled coils. It is also noted that condensers having diagonal arrows are uni-controlled variable tuning condensers. while condensers having d agonal lines without arrow heads are adjustable condensers which remain fixed after the required adjustment has been made.

Referring to Figs. 2, 3, and 4. which illustrate a 5 preferred mechanical arrangement of the receiver of Fig. l, the apparatus is mounted upon a chassis pan ind which is of aconventional. form having the sides and ends thereof bent downward at a right angle to form a raised horizontal base. Upon the upper side of the chassis is a metal frame ml which supports the variable tuning condensers i8, 28, '29, and 62. These condensers are not shown in detail in Fig. 2. since they are of a conventional form mounted in line upon a shaft use for uni-control operation. The end of the shaft has fastened thereto a gear W3 adapted to be driven by a pinion iill which is mounted upon a control shaft 005. A knob IDS is fastened to the end of control shaft 005 outside the chassis pan. This knob controls different properties of the selective network, as will be described hereinafter.

I Situated in line along the center of the chassis pan is a row of shielding cans H9, H0, iZl, I22, and 123, mounted above the pan. Each of these shielding cans encloses one of the selective coupling systems of the receiver. Can H9 encloses coupling coil 28 and secondary coil E9 of the preselector; cw A20 encloses elements 2t, 2t, and

28 of transformer 25; can I H encloses elements 48, 49, 50, and 5|, of transformer 55; can I22 encloses elements 55, 56, 57, and 50, of transformer 54; and can I23 encloses elements 65, 06, 61, and 68 of transformer 64.

In the embodiment illustrated, the immovable secondary coil of each of the selective coupling systems is mounted coaxially in the shielding can enclosing it, while the movable primary coil is mounted coaxially below it and is adapted to be moved in an axial direction on a mounting lug and core extending through an opening in the chassis pan. The, two inductively coupled coils of each of these coupling systems are mounted coaxially within a coaxial cylindrical shield which extends axially beyond both coils. Fig. 3 shows how'this adjustment of eachpf the movable primaries is obtained. This figure shows the position of shaft I05 under the chassis pan and also shows in section the shielding can I22 containing the fixed secondary coil 56 and the movable primary coil 55 of intermediate-frequency coupling transformer 54. The construction and mechanical arrangement of the selective coupling systems, as described herein, have proved successful in practice, but other equivalent mechanical arrangements and specific relative modes of motion of the coils may be employed.

For simplicity of illustration, the only shielding can shown in Fig. 3, is can I22, although it will be understood that the remaining cans enclosing movable coils are similarly mounted. In can I22 the secondary coil 56 is mounted on a core supported from within the top of the can. Primary coil 55 is supported, coaxially with respect to coil 56, on a core carrier by mounting lug I IS! The opposite end of the mounting lug is supported on a rigid cross member I I2 along the underside of the chassis pan directly beneath all of the cans II9, I20, I2I, I22, and I23. The cross member I I2 is supported at its ends by two crank supports H3 and I I3. These crank supports are mounted on a shaft II4 along the rear of the chassis pan, parallel to the cross member H2. The vertical position of cross member H2 is determined by a rigid projecting lug I II attached at one end to member H2, and engaging at the other end a notch I29 in a bell crank I00, the

latter being mounted on a shaft I09. To counterbalance the weight due to projecting arm III acting on one side of thebell crank, the opposite side of the bell crank is attached to a counterweight I I0.

There is fastened to the inner end of the control shaft I05 a disc I0'I which engages a slot I30 in bell crank I08. .The shaft I 05 is movable in an axial direction whichis a degree of freedom termed translation, so that the position of the bell crank and the resulting elevation of coil 55 (and of all other coils movable therewith) is determined by the axial position of the shaft I05. Movable coils 2|, 40, 55, and 65, are supported on mounting lugs, similar to lug I I5, which are, in turn, mechanically secured to the cross member I I2 so that the coils move in unison to like extents by translation of shaft I 05.' It is contemplated, however, that in other modifications the coils may be moved to different extents, depending upon the specific design.

The handle or knob I06, secured to the end first, by rotation about its axis, and second, by translation along its axis. When pinion I04 is engaged with gear I03, movement of the knob in the first degree of freedom (rotation) adjusts the tuning of the receiver by simultaneous move- 5 ment of all of the variable tuning condensers, which are electrically similar and mounted for simultaneous operation upon the shaft I02. Motion of the knob in the second degree of freedom (translation) simultaneously varies the coupling 10 between coils 2i and I9, 40 and 50, 55 and 56, and 65 and 65, as explained above, and thereby adjusts the band width of the selector and hence I the selectivity of the receiver. With this construction the receiver can be accurately tuned 15 only when coupling of the selectors is substantially a minimum, corresponding to minimum band width and maximum selectivity. This result is accomplished by an interlocking means comprising the mechanical mounting of pinion 20 I05 and gear I03 so that the two are automatically disengaged when the selector band width is adjusted.

In operation, first the receiver is tuned by rotation of the single tuning knob I06, in the con- 25 dition of greatest selectivity, which corresponds to minimum band width. On reaching the exact tuning adjustment for receiving the desired signal, the tuning mechanism may be disengaged by drawing out the knob I06, and the selector band 30 width expanded to respond more uniformly to the modulation side bands of the desired signal carrier. This provides any degree of fidelity permissible underthe particular conditions of signal interference or noise existing at the time. 35 The mechanism is so designed that in drawing the knob I06 outward, the expansion is proportional to the outward displacement of the knob. When it is desired again to tune the receiver, the knob I06 must be pushed in to 40 engage the tuning gears, thus the user or broad Y cast listener or operator is constrained always to tune the receiver when it is electrically in the most advantageous condition for that operation. 4:) Each variable coupling transformer I3, 46, 54, and 64, constitutes a band-pass selective network, or band selector, whose selectivity and band width are adjustable. Likewise, the combination of all these transformers constitutes such a net- 50 work. In each transformer, it, is preferable to vary the coupling between half optimum and twice optimum. Less or greater couplings substantially reduce the gain of the system in the center of the hand, without compensating ad- 55 which may be of the order of six kilocycles. The reception of both sidebands with uniform responsiveness therefore requires a band selector whose band width is at least double the modulation- 7 frequency band, and an audio system whose band width is-at least equal to the modulation-frequency band. In the circuit of Fig. l, the band selector. may be adjustable from a minimum width of 3 kilocycles to to a maximum width of 12 kilocycles, while the audio system, including the tone control, may be adjustable from a minimum width of 1.5 kilocycles to a maximum width of 6 kilocycles. The intermediate-frequency band selectors may have a fixed center frequency of 175 kilocycles. The receiver as a whole may be tunable over the broadcast range of 550 to 1500 kilocycles.

From the point of view of the antenna-ground system, looking through the modulator or frequency changer, transformers 46, 5d, and 64 are, in effect, band selectors whose center frequency is increased by the amount of the oscillator frequency tuned by variable condenser 52 which controls the action of the frequency changer. The combination of the frequency changer and any or all of transformers l6, 5%, and 6d, constitutes a band-pass selective network or band selector whose center frequency is tunable in operation by variable condenser #32.

From the point of view of transformers d6, 56, and 66, looking backward through the modulator or frequency changer, the received carrier frequency is effectively changed to the center frequency of the band selectors 66, 5d, and 6%, by tuning the condenser :32.

Using the preferred system according to this invention, (.1) the width of the band can be adjusted to substantially less than double the highest modulation frequency; (2) the center of the band can be tuned to a desired carrier signal, by tuning the band-pass carrier amplifier which includes the band-pass selector; and (3) the width of the band can then be readjusted to substantially double the highest required audio-frequency, or modulation frequency, without detuning the center of the band. Stated with reference to the audio-frequency amplifier system (which is a band-pass modulation-frequency amplifier), (1) the selector band width can be adjusted tosubstantially less than double the upper cutoff frequency of the modulation-amplifier band; (2) the tunable frequency changer can be tuned to shift a desired carrier frequency to the center frequency of the selector band; and (3) the width of the selector band can then be readjusted to a maximum value substantially double the upper cutofi frequency of the modulationamplifier band. The modulation frequencies in the output of the audio amplifier may then be utilized in any well known manner, such as by a loud speaker.

It will be noted that the input circuit of each of the double-tuned coupling transformers td, 5d and til, each of which constitutes of itself a band selector, includes the output circuit of a preceding vacuum tube, while the output circuit of each of these coupling arrangements includes the input circuit of a succeeding vacuum tube. By virtue of the fact that these coupling transformers are double tuned, however, and that the input and output conductances of the associated tubes are inherently substantially zero, the conductances of the associated tubes will have no appreciable effect on the band widths of the selectors or the mean frequencies thereof, which would otherwise be the case if only one of the circuits of the coupling transformer were tuned. Thus, variations in the input and output conductances of the tubes in operation, as by the effects of automatic volume control, band width of the selectors or the tuning of the system. Where in the accompanying claims the words substantially zero" are employed with reference to tube conductance, they are intended will have no effect on the to mean a value which is so low that the conductance will have no appreciable effect.

The responsiveness of the selector, i. e., the gain in the carrier frequency amplifier, is incidentally changed somewhat by varying the coupling in transformers i3, 46, and 54. This is compensated by the automatic volume control. Otherwise, this variation would, in the aggregate, be sufiicient to be objectionable.

The variable attenuator and tone control arrangement is illustrated in Fig. 4. The attenuator is the potentiometer 80 of Fig. 1, which, as illustrated in Fig. 4, is of the rotary shaft type. The rotary shaft of the potentiometer is the tubular shaft I28 (for adjusting the attenuation) and is keyed to an inner shaft l2'i extending through the front of the chassis pan. The outer end of shaft I21 is provided. with a knob M6, for manual adjustment.

The tone-control resistor 82 of Figs. 1 and 4, is also of the rotary shaft type. The rotary shaft of variable resistor 82'has attached thereto a pinion I H which is engaged by a cylindrical rack H6 secured to the shaft iZ'l.

The knob I26 and its shaft I2! are movable in the same degrees of freedom as knob I06 and its shaft I05. Rotation of knob I26 varies the attenuation by rotation of potentiometer 80 andconstitutes a volume level control. Axial displacement, or translation, of knob E26, rotates pinion ill which varies the resistance of 82 and constitutes a tone control. There is no interlocking in this case, because the keyway in shaft i2! is always engaged, and hence any desired combination of volume level and tone may be secured by manipulation of knob are.

The tone control is preferably arranged to expand the band width of the audio system, or audio-frequency amplifier 8i, when knob H6 is pulled outward, in substantially the same manner that the selectivity control is arranged to expand the band width of the band selector, when knob M16 is pulled outward. Thus, the band widths of the carrier amplifier and of the audio amplifier are expanded by axial motions of the knobs in the same direction. The amount of expansion in the audio system should be only half as great as in the selector, because the expansion of the selector must include both sidebands. This symmetrical mechanical and electrical arrangement greatly facilitates the proper operation of these knobs by the user. In fact, a mechanical uni-control link may be employed for causing the same amount of axial motion of both controls or knobs.

The remaining apparatus is mounted on the chassis pan as indicated. The can M8 encloses coils i5, i6, and if, of the antenna coupling system. Can lZd encloses elements 37, 38, all, it, and 63 of the oscillator system. Can E25 encloses elements 88, 89, 90, and 9! of transformer 86. Vacuum tubes 23, 26, ii, 53, ii, and 53, oocupy the positions shown in Fig.2.

While the foregoing description of a preferred embodiment of this invention relates to a superheterodyne radio receiver, the invention is equally applicable to other types such as the tunedradio-frequency amplifier; and likewise is applicable to any type of coupling system, whether it has the simple inductive or capacitive type, or a compound type involving both.

We claim:-

1. The method of operating a modulated-carrier receiver having a tunable frequency changer and a band-pass selector, which comprises first said amplifier band, tuning the center of said selector band to a desired carrier frequency, then readjusting the width of said selector band to substantially double said upper cutoff frequency without detuning the center of said selector band, and finally utilizing all the received modulation frequencies within said amplifier band.

3. The method of operating a modulated-carrier receiver having a tunable frequency changer, a band-pass selector, a detector,- and a band-pass modulation-frequency amplifier, which comprises first adjusting the width of said selector band to substantially less than double the upper cutoff frequency of said amplifier band, tuning said frequency changer to shift a desired carrier frequency to the center frequency of said selector band, then readjusting the width of said selector band to substantially double said upper cutoff frequency without altering said center frequency, and finally utilizing all the received modulation frequencies within'said amplifier band;

4. A radio receiver having a modulated-carrier amplifier and an audio amplifier, and including two control knobs, means permitting both rotational and translational motion of each knob, means for varying the band width of said carrier amplifier between limits of substantially less than double the highest modulation frequency desired to receive and substantially double said frequency by one said motion of one said knob, and means for varying the band width of said audio amplifier in the same sense by the corresponding motion of the other said knob in corresponding directions.

5. A radio receiver having a carrier amplifier and an audio amplifier, and including two similar knobs symmetrically located on said receiver, means permitting both rotational and axial motion of each knob, means for tuning the carrier amplifier by rotation of one knob, means for independently varying the band width of the carrier amplifier by axial motion of the same knob, means for varying the gain of the audio amplifier by rotation of the other knob, means for independently varying the band width of the audio amplifier by axial motion of the other knob, said band widths being increased by axial motions in the same direction.

6. The method of operating a superheterodyne receiver having a plurality of intermediate-frequency selectors each including two inductively coupled coils, which comprises first permanently tuning said coils to the intermediate frequency, adjusting the coupling between the coils of said selectors to a value substantially less than optimum, tuning the receiver to a desired signal, and finally increasing the coupling to a value substantially greater than optimum.

7. In a superheterodyne receiver, a plurality of intermediate-frequency selectors each including two inductively coupled coils tuned to the intermediate frequency, selectivity control means for simultaneously varying the coupling in the selectors, tuning control means for tuning the receiver, and interlocking means for disengaging the tuning control except when the selectivity control is adjusted for substantially minimum coupling.

8. In a modulated-carrier receiver, a band-pass selector, tuning control means for tuning the receiver over a range of frequencies, selectivity 10 control means for independently adjusting the band width of the selector, and interlocking means for disengaging the tuning control when the selectivity control is adjusted for substantially greater than minimum band width.

9. In a superheterodyne receiver, a frequency changer, an intermediate-frequency band-pass selector, tuning control means for tuning the frequency changer over a range of frequencies, selectivity control means for adjusting the band width of the selector, and interlocking means for disengaging the tuning control when the selectivity control is adjusted for substantially greater than minimum band Width.

10. In a modulated-carrier receiver, a bandpass selector, a single knob for controlling all the selective properties of the selector, tuning control means for adjusting the center frequencyof the band by rotation of the knob, selectivity control means for independently adjusting the band width of the selector by motion of the knob in the axial direction, and interlocking means for disengaging the tuning control when the knob is adjusted for substantially greater than minimum band width. 3',

11. In a superheterodyne receiver, a frequency changer, an intermediate-frequency band-pass selector, a single knob for controlling all the seiective'properties of the receiver, tuning control means for tuning the frequency changer by 40 rotation of the knob, selectivity control means for independently adjusting the band width of the selector by motion of the knob in the axial direction, and interlocking means for disengaging the tuning control when the knob is adjusted for substantially greater than minimum band width.

12. The method of operating a tunable modulated-carrier receiver having an adjustable bandpass selector, which comprises contracting the width of said band substantially to a minimum value, to permit highly selective reception of signals in closely spaced frequency channels, and to facilitate exact tuning to a desired signal; exactly tuning the receiver in said highly selective condition to the desired signal; and then expanding the width of said band to improve the fidelity of reception to the greatest extent consistent with avoiding interference, while maintaining the tuning unaltered, and while automatically compensating for any change of responsiveness incidental to the expanding of the band width.

13. In a superheterodyne receiver, a plurality of intermediate-frequency selectors each including two inductively coupled coils tuned to the intermediate frequency, selectivity control means for simultaneously varying the coupling in the selectors, tuning control means for tuning the receiver, and interlocking means for preventing the tuning of the receiver by the tuning control except when the selectivity control is adjusted for substantially minimum coupling.

14. In a modulated-carrier receiver, a bandpass selector, tuning control means for tuning the receiver over a range of frequencies, selectivity control means for independently adjusting the band width of the selector, and interlocking means for preventing the tuning of the receiver by the tuning control when the selectivity control is adjusted for substantially greater than mini-' mum band width.

15. In a superheterodyne receiver, a frequency changer, an intermediate-frequency band-pass selector, tuning control means for tuning the frequency changer over a range of frequencies, selectivity control means for adjusting the band width of the selector, and interlocking means for preventing the tuning of the receiver by the tuning control when the selectivity control is adjusted for substantiallylgreater than minimum band width.

16. In a modulated-carrier receiver, a bandpass selector, a single knob for controlling all the selective properties of the selector, tuning control means for adjusting the center frequency of the band by rotation of the knob, selectivity control means for independently adjusting the band width of the selector by motion of the knob in the axial direction, and interlocking means for preventing the tuning of the receiver by the tuning control when the knob is adjusted for substantially greater than minimum band width.

1'7. In a superheterodyne receiver, a frequency changer, an intermediate-frequency band-pass selector, a single knob for controlling all the selective properties of the receiver, tuning control means for tuningthe frequency changer by rotation of the knob, selectivity control means for independently adjusting the band width of the selector by motion of the knob in the axial direction, and interlocking means for preventing the tuning of the receiver by the tuning control when the knob is adjusted for substantially greater than minimum band width.

18. The method of operating a modulated-carrier receiver having two simultaneously tunable circuits adjustably coupled to provide a bandpass selector, which comprises first adjusting the width of said band to substantially less than double the highest modulation frequency desired to receive, then simultaneously tuning said circuits to resonance with a desired carrier frequency whereby said desired carrier frequency is at the center of said band, and then readjusting the width of said band to substantially double said highest modulation frequency without altering said center frequency.

19. The method of operating a. modulated-carrier receiver having two simultaneously tunable circuits adjustably coupled to provide a bandpass selector, a detector, and a band-pass modulation-frequency amplifier which comprises first adjusting the coupling of said circuits to reduce the Width of said selector band to substantially less than double the upper cut-ofi frequency of said amplifier hand, then simultaneously tuning said circuits to bring the center of said selector band to a desired carrier frequency, and then readjusting said coupling to broaden the width of said selector band to substantially double said upper cut-off frequency without detuning the center of said selector band, and finally utilizing all the received modulation frequencies within said amplifier band. 1

20. In a modulated-carrier receiver, a modulated-carrier amplifier including a vacuum tube having substantially zero conductance between its output terminals and a band selector including a pair of tuned circuits coupled by mutual inductance and connected between said terminals,

means for varying the mutual inductance between said circuits to adjust the selector band width independently of said conductance and incidentally varying the responsiveness of said selector, and automatic volume control means for compensatingthe variation of responsiveness.

,21. In a superheterodyne receiver, an interme diate-frequency amplifier including a first vacuum tube having substantially zero conductance between its output terminals, a second vacuum tube having substantially zero conductance between its input terminals, and a band selector having two tuned circuits coupled by mutual inductance and connected respectively between said output terminals and between said input terminals, means for manually adjusting said mutual inductance and thereby varying the selector band width independently of said conductances and incidentally varying the responsiveness of said' selector, and automatic volume control means for varying the mutual conductance of at least said first tube and thereby compensating the variation of responsiveness.

22. In a modulated-carrier receiver, a band- 4 pass selector, tuning control means adjustable to operative position for tuning the receiver over a range of frequencies, selectivity control means for adjusting the band width of the selector, and means operative upon adjustment of the tuning control means to operative position to adjust the selectivity control for substantially minimum band width.

HAROLD A. WHEELER. NELSON P. CASE. 

